The heart is a hollow muscular organ having four pumping chambers separated by four heart valves: aortic, mitral (or bicuspid), tricuspid, and pulmonary. Heart valves are comprised of a dense fibrous ring known as the annulus, and leaflets or cusps attached to the annulus.
Prosthetic heart valves can be used to treat cardiac valvular disorders. The native heart valves (such as the aortic, pulmonary, tricuspid, and mitral valves) serve critical functions in assuring the forward flow of an adequate supply of blood through the cardiovascular system. These heart valves can be rendered less effective by congenital, inflammatory, or infectious conditions. Such conditions can eventually lead to serious cardiovascular compromise or death. It is possible to surgically repair or replace the valve during open heart surgery, where a prosthetic valve is sutured in place, but such surgeries are time-consuming, dangerous and prone to complication.
Transvascular and transapical techniques can be used for introducing and implanting a prosthetic heart valve using a flexible catheter in a manner that is less invasive than open heart surgery. In these techniques, a prosthetic valve can be mounted in a crimped state on the end portion of a flexible catheter and advanced through a blood vessel of the patient until the valve reaches the implantation site. The valve at the catheter tip can then be expanded to its functional size at the site of the defective native valve, such as by inflating a balloon on which the valve is mounted. Alternatively, the valve can have a resilient, self-expanding stent or frame that expands the valve to its functional size when it is advanced from a delivery sheath at the distal end of the catheter. These are sutureless techniques which greatly reduces the procedure time.
Balloon-expandable valves can be used for treating heart valve stenosis, a condition in which the leaflets of a valve (e.g., an aortic valve) become hardened with calcium. The hardened leaflets provide a good support structure on which the valve can be anchored within the valve annulus. Further, the catheter balloon can apply sufficient expanding force to anchor the frame of the prosthetic valve to the surrounding calcified tissue. There are several heart conditions, however, that do not involve hardened valve leaflets but which are still desirably treated by valve replacement. For example, aortic insufficiency (or aortic regurgitation) occurs when an aortic valve does not close properly, allowing blood to flow back into the left ventricle. One cause for aortic insufficiency is a dilated aortic annulus, which prevents the aortic valve from closing tightly. In such cases, the leaflets are usually too soft to provide sufficient support for a balloon-expandable prosthetic valve. Additionally, the diameter of the aortic annulus may continue to vary over time, making it dangerous to install a prosthetic valve that is not reliably secured in the valve annulus. Similar problems can occur with other heart valves as well. For example, mitral insufficiency (or mitral regurgitation) involves these same conditions but affects the mitral valve.
Self-expanding prosthetic valves can suffer from other problems. For example, if a self-expanding prosthetic valve is placed within the patient's defective heart valve (e.g., the aorta or mitral valve), it could continues to exert an outward force on the valve annulus. This continuous outward pressure could cause the valve annulus to dilate further, exacerbating the condition the valve was intended to treat. Additionally, when implanting a self-expanding valve, the outward biasing force of the valve's frame could cause the valve to be ejected very quickly from the distal end of a delivery sheath. This makes delivery of the valve very difficult and dangerous to the patient.
The size of the prosthetic valve to be implanted into a patient can also be problematic when treating aortic or mitral insufficiency. Specifically, the size of a prosthetic valve used to treat aortic or mitral insufficiency would generally need to be larger than a prosthetic valve used to treat aortic or mitral stenosis. This larger valve size can make the delivery procedure much more difficult and dangerous to the patient.
Another potential issue with sutureless valves is valve migration. For example, when an aortic prosthetic valve is implanted, 100-200 mmHg pressure loads on the aortic valve immediately. The pressure times the valve surface area produces a substantial load force on the prosthetic valve and could cause valve migration towards the aortic arch. Another potential cause of valve migration is a tilted valve landing. When tilted, the prosthetic valve will have a larger surface area facing the blood flow, which could push the prosthetic valve into the aorta.
Treatment of the mitral valve can present additional challenges, and methods and an apparatus appropriate for the aortic valve may not be well suited for use with the mitral valve. For instance, the mitral valve includes clusters of chordae tendineae extending from the valve leaflets to the walls of the ventricle that may interfere with placement of the prosthesis. The shape of the mitral valve, rather than being circular and uniform like the aortic valve, can be an oval or kidney-like shape that may not be well suited for supporting conventional stents of cylindrical configuration. Further, whereas the aortic valve annulus is often entirely surrounded by muscular tissue, the mitral valve annulus may be bounded by muscular tissue on the outer (posterior) wall only. The anterior side of the mitral valve annulus is bounded by a thin vessel wall adjacent the left ventricular outflow tract (“LVOT”), which must remain open to allow blood to pass into the aorta. As a result, the stent-type fixation may not be suitable for the mitral valve because the anterior side of the native valve has insufficient radial strength and can distort, risking occlusion of the left ventricular outflow tract. Moreover, mitral valve disease often is accompanied by (or caused by) gradual enlargement of the native annulus and/or the left ventricle. Thus, treatment approaches which rely upon radial engagement with or outward compression against the native annulus are subject to failure as the size and shape of the annulus changes.
There is a need for improved methods, systems, and apparatus for delivering expandable prosthetic heart valves (e.g., balloon-expandable or self-expandable prosthetic valves). Embodiments of the methods, systems, apparatus, devices, components, etc. disclosed herein can be used to replace native heart valves even when they do not have calcified leaflets (e.g., aortic valves suffering from aortic insufficiency).